1. Field of the Invention
This invention relates generally to a method and apparatus utilized in hydrocarbon exploration. More specifically, the invention relates to formation testing tools. Even more particularly, the present invention is directed to methods and apparatus for performing formation testing while drilling.
2. Background and Related Art
Geologists and geophysicists are interested in the characteristics of the formations encountered by a drill bit as it is drilling a well for the ultimate production of hydrocarbons from the earth. Such information is useful in determining the correctness of the geophysical data used to choose the drilling location and in choosing subsequent drilling locations. In horizontal drilling, such information can be useful in determining the location of the drill bit and the direction that drilling should follow.
Such information can be derived in a number of ways. For example, cuttings from the mud returned from the drill bit location can be analyzed, or a core can be bored along the entire length of the borehole. Alternatively, the drill bit can be withdrawn from the borehole and a “wireline logging tool” can be lowered into the borehole to collect data or otherwise determine formation characteristics. In still another approach, called “measurement while drilling” (“MWD”) or “logging while drilling” (“LWD”), tools are included in the drill string that collect formation data while the drill bit remains in the borehole.
One type of formation testing tool measures formation pressure, which can be used for a variety of purposes, including computing the permeability and porosity of the formation. A conventional such formation testing tool operates in the wireline environment. It is lowered into the well to a depth where formation testing is desired. Before the wire line tool can be lowered, however, the entire drill string must be removed from the borehole. This process, known as “tripping” is a laborious and time consuming process by which the drill string, which may be miles long, is removed from the hole, pipe section by pipe section. After the formation tester has been lowered to the appropriate depth by means of a wireline, the borehole interval adjacent to the tester must be packed off and isolated from the drilling fluid that remains in and fills the borehole so that accurate reading of the formation pressure can be obtained. With the pressure recorded, the tool is retrieved to the surface for analysis and the drill string is then reassembled and replaced in the borehole, section by section. As well be understood, conducting formation tests via a wireline tool is time consuming and costly, given that costs of drilling a well may be thousands of dollars per hour.
As mentioned above, testing the formation using a tester incorporated into the drill string is desirable in that the drill string does not need to be removed to conduct the test. However, there are various complications associated with conventional such apparatus. For example, in certain such testers, the flow of drilling fluid must be stopped in order to measure the formation pressure or take a sample of the formation fluid. When this occurs, without the flow of constantly moving drilling fluid, the bottom hole assembly can become stuck in the hole, necessitating a costly and time consuming procedure to free the stock tool. Furthermore, mud turbine generators are sometimes employed in the bottom hole assembly as the means of supplying electrical power needed to actuate the formation tester. In such tools, stopping the flow of drilling fluid therefore prevents the tool from generating the needed electrical power, and power to operate the formation tester must be supplied by other means, such as batteries which, in certain instances, may be less reliable or otherwise less desirable. Other problems and shortcomings are associated with present day formation testers.
For example, certain conventional formation testers employ a extendible probe that extends from the tool to engage the borehole wall in order to conduct the fluid test or sampling. In certain instances, however, particularly when drilling a horizontal well, the orientation of the tool may be such that the probe extends out of the tool on the low side of the hole. When this occurs, the extending probe may be subjected to detrimental loading as the piston extends and contacts the borehole. Further, there are many instances during which the extending probe will engage the borehole wall at an angle, rather than being normal to the wall. When this occurs, the seal necessary for properly extracting and measuring formation fluid pressure is difficult, if not impossible, to achieve.
Accordingly, there remains a need in the art for a formation testing apparatus that may be employed in a drill string to conduct reliable formation testing. Ideally, such apparatus would not require that the flow of drilling fluid be cut off so as to prevent the bottom hole assembly from sticking to the borehole and permit the formation tester to be powered by the flow of drilling fluid. Further, it would be preferable if the sensed data and other measurements could be communicated to the surface via mud pulse telemetry, which relies on the flow of drilling fluid. A formation tester that insures that an extending probe contacts the borehole wall substantially normal to the wall, rather than at an angle, and which protects the probe from excessive bending moments and other excessive forces would be particularly welcomed by the industry.